1. Field of the Invention
The invention generally relates to recovering metallic species in a vapor state, and, more specifically, to condensing vapors of metals to achieve relatively high recovery of the same.
2. Description of Related Art
Magnesium is the lowest-density engineering metal, with alloys exhibiting outstanding specific stiffness and strength. It exhibits a relatively low boiling point among metals, such that several processes produce it as a vapor, which enables in-line distillation. However, it also exhibits the highest vapor pressure at its melting point of all metals: nearly 2 torr. This makes it difficult to condense magnesium vapor as a liquid, because even with perfect mass transfer, significant magnesium remains in the vapor phase at its melting point, so one must control temperature very carefully to avoid either leaving significant magnesium in the vapor phase or producing solid metal particles. A liquid metal product is advantageous over solid product because it is much easier to remove a liquid from the process and cast it into ingots or parts, alloy it with other metals, or form other useful products than would be the case for solids.
Condenser apparatus such as those of Allen (U.S. Pat. No. 2,514,275) and Pidgeon (U.S. Pat. No. 2,837,328), which have been the norm in the magnesium industry for decades, produce only solid magnesium. A liquid magnesium condenser by Schmidt (U.S. Pat. No. 3,505,063) produces magnesium-aluminum alloys which are suitable for aluminum alloy production, but do not contain sufficient magnesium for magnesium-base alloys.
A device by Schoukens et al. (U.S. Pat. No. 7,641,711) condenses liquid magnesium from vapor with magnesium partial pressure of 0.7-1.2 atmospheres (70-120 kPa). This device recovers magnesium as a liquid for processes such as the Magnatherm metallothermic magnesium reduction technology (see U.S. Pat. Nos. 2,971,833 and 4,190,434), which can produce magnesium at that pressure. However, at the high temperature over 1800° C. required for metallothermic production near atmospheric pressure (see U.S. Pat. Nos. 5,090,996 and 5,383,953), other elements such as manganese, iron, nickel and copper are volatile and can enter the magnesium product as impurities. And Schoukens' condenser is not as effective when input magnesium partial pressure is below 0.7 atmospheres (70 kPa), e.g. the Pidgeon process (see U.S. Pat. No. 2,387,677) and similar low-pressure metallothermic reduction processes. Schmidt's patent (U.S. Pat. No. 3,505,063) gives another reason for difficulty in producing liquid magnesium from a metallothermic reduction vapor stream, which is the variable or “pulsed” rate of magnesium entry into the condenser and its vapor pressure, making it very difficult to control condenser temperature tightly enough to reliably produce liquid magnesium.
The Solid Oxide Membrane (“SOM”) electrolysis process (see U.S. Pat. Nos. 5,976,354 and 6,299,742) shown in FIG. 1 efficiently produces pure oxygen gas and metals from metal oxides. When producing magnesium by SOM electrolysis (see, e.g., A. Krishnan, X. G. Lu and U. B. Pal, “Solid Oxide Membrane Process for Magnesium Production directly from Magnesium Oxide,” Metall. Mater. Trans. 36B:463, 2005), it is convenient to operate the electrolysis cell above the 1090° C. boiling point of magnesium, as operating at this temperature promotes high ionic conductivity of the zirconia SOM and purifies the magnesium product by distillation (as shown in FIG. 1). Unfortunately, when the magnesium product partial pressure is above a threshold, it reacts with and damages the zirconia SOM; that threshold equilibrium magnesium partial pressure is approximately 0.15 atm at 1150° C. and 0.33 atm at 1300° C. (15 and 33 kPa respectively). Unlike metallothermic reduction, in SOM Electrolysis the electric current determines the rate of magnesium production. And because it is easier to control the current in SOM electrolysis than the reaction rate in metallothermic processes, there is far less fluctuation in magnesium partial pressure and temperature at the condenser. This facilitates (but is not necessary for) operating a liquid condenser for this process, at whose magnesium partial pressure the condenser of Schoukens et al. is not effective as mentioned above. On the other hand, it is difficult to shut down and restart a self-heated electrolysis cell, such as SOM electrolysis of magnesium, due to salt freezing and other phenomena. Thus it is important for a magnesium condenser for this process to be able to operate continuously without periodically shutting off.